A number of types of dot matrix print heads are known. A typical dot matrix print head comprises a magnetic gap of a magnetic circuit including a permanent magnet and a solenoid, an armature located in the magnetic gap and supported by spring means, and a wire connected to the armature by way of an arm. Normally, the spring means is biased by the magnetic flux acting upon the armature. The magnetic flux in the magnetic gap may be interrupted by energizing the solenoid and canceling the magnetic flux of the permanent magnet with the magnetic flux of the solenoid, whereby the wire is axially driven towards paper by the restoring force of the spring means. Normally, a plurality of wires are used in a straight row or, alternatively, in a staggered arrangement so as to achieve a desired print by a matrix of wires impacting upon the paper.
Such a print head is generally required to be light in weight so that the print head would not produce excessive inertia force as a carriage carrying the print head travels across the paper. Also, the speed of the reciprocating axial motion of print wires is desired to be as fast as possible for greater printing speed. This may be achieved by increasing the magnetic flux of the magnetic circuit and reducing the effective mass of the assembly including the spring means, the armature, the arm and the wire.
In order to increase the magnetic flux of the magnetic circuit, the permanent magnet must be increased in size and, accordingly, the size of the armature must be increased so as to prevent the saturation of the magnetic flux density in the armature. For instance, U.S. Pat No. 4,389,127 teaches a print head in which a high printing speed is achieved through advantageous structure of a magnetic circuit. However, to ensure sufficient mechanical strength to the sheet spring supporting the armature whose inertia force is substantial, crossed sheet members are used as the spring means. Therefore, the structure of this print head is fairly complex and expensive to manufacture.
U.S. Pat No. 4,225,250 teaches a simple print head using a ring permanent magnet and a plurality of hammers each carrying a print wire at its free end. Since each of the hammers consists only of an inwardly projecting tongue of a sheet spring member and serves the functions of a spring member, an armature and an arm, this print head is highly simple in structure and light in weight. However, absence of an armature with a certain cross sectional area limits the maximum magnetic flux which may be utilized because the thin hammer can carry very limited magnetic flux therethrough, and its printing speed is limited accordingly.
Normally, an armature is brazed to the free end of a sheet spring, but brazing is a fairly expensive process and may not ensure uniform mechanical strength. When the armature is welded to the sheet spring for instance by a laser beam, craters are formed in the surface of the sheet spring facing the first pole surface and cause a substantial magnetic resistance in the interface between the sheet spring surface and the first pole surface.